Vacuum electron device electrodes and components manufactured from highly oriented pyrolytic graphite (HOPG)

ABSTRACT

Components for use in vacuum electron devices are fabricated from highly oriented pyrolytic graphite (HOPG) and exhibit excellent thermal conductivity, low sputtering rates, and low ion erosion rates as compared to conventional components made from copper or molybdenum. HOPG can be reliably brazed by carefully controlling tolerances, calculating braze joint material volume, and applying appropriate compression during furnace operations. The resulting components exhibit superior thermal performance and enhanced resistance to ion erosion and pitting.

RELATED APPLICATIONS DATA

This application claims priority pursuant to 35 U.S.C. §119(e) to U.S.provisional patent application Ser. No. 61/495,213 filed Jun. 9, 2011,the subject matter of which is incorporated by reference herein in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to improved vacuum electron deviceelectrodes and other microwave components manufactured from highlyoriented pyrolytic graphite (HOPG).

2. Description of Related Art

The components used in the manufacture and assembly of vacuum electrondevices, such as RF output vanes, RF straps, traveling wave tube (TWT)slow wave circuits and output lines, Linear Beam cavity drift tubes,meander lines, and thyratron electrodes, among others, are known in theart. These components are very typically manufactured from copperbecause of its good thermal conductivity, relative low cost, andmachineability. For higher peak power densities, the copper is oftencladded with tungsten or molybdenum.

For example, FIGS. 1 a through 1 c depict an internally cooled coppervane 102 from an AEGIS-style microwave amplifier, typical of the priorart, that failed due to cavitation erosion. As evident from the figures,the cooling channel 104 in the vane 102 shows cavitation damage inregion 106 and pinhole breakthrough that caused failure of this device.

Up to now, suitable alternatives to copper and copper clad formanufacturing microwave electrodes have not been known or been readilyavailable. While certain materials are known to have high thermalconductivity and other desirable properties, they have not beenconsidered for use as microwave components because of the perceiveddifficulty of machining, brazing, and otherwise adapting them formicrowave applications. For example, it is known that highly-orientedpyrolytic graphite (HOPG) has a very high thermal conductivity over fourtimes that of copper. However, it is expensive, it is a soft material,like mica, that is difficult to machine, it is not readily susceptibleto electrical discharge machining (EDM), and it has been perceived asdifficult to reliably braze. In addition, pyrolytic graphite is highlyanisotropic. The physical properties vary widely, dependent on thechosen plane of its hexagonal crystal structure. It would thus beadvantageous to demonstrate methods by which HOPG can be employed tomanufacture reliable and high-performance microwave components thatovercome the disadvantages of the prior art.

SUMMARY OF THE INVENTION

An embodiment of the present invention comprises a component for use ina vacuum electron device comprising a first portion constructed fromhighly oriented pyrolytic graphite (HOPG) and having a high thermalconductivity. The component also includes a second portion constructedfrom a metal that is brazed to the HOPG portion, the braze jointproviding both mechanical and thermal coupling between the HOPG portionand the metal portion.

In some embodiments, the metal portion is constructed from copper,molybdenum, kovar, or a copper-molybdenum alloy. The braze material maycomprise a gold-copper alloy in some embodiments.

In some embodiments of the present invention, the component manufacturedfrom HOPG and metal exhibits a thermal conductivity that is higher thana similar component made from metal. The component may comprise a crossfield amplifier (CFA) vane structure, a magnetron structure, an electrontube anode or cathode structure, or any other such component known inthe art.

In some embodiments, the metal portion may be milled to create a cavitywithin which the HOPG portion fits in order to facilitate brazing.

A method of manufacturing a microwave component in accordance with anembodiment of the present invention includes the steps of machining ametal portion to create a brazing surface and machining an HOPG insertto fit closely within the machined metal portion. In some embodiments,the metal portion may be machined to leave a thin web of material thatprovides a brazing surface. The machining may be performed using adiamond blade. A volume of braze alloy is calculated to fit within thespace between the machined metal portion and the HOPG insert. Theassembly is then heated in a brazing furnace to create a mechanical andthermal bond between the metal portion and the HOPG portion. The brazingoperation may be performed in a vacuum or in a nitrogen atmosphere.

In some embodiments of the claimed method, the process of assembling thecomponent includes covering the HOPG portion with a metal foil that isthen bonded through active brazing. The metal foil may comprise a foilmade from copper, copper-molybdenum, molybdenum, kovar, or othersuitable metals known in the art.

In some embodiments of the claimed method, the HOPG and metal assemblyis wrapped in nichrome wire in order to create a clamp that maintainscompressive force on the assembly during the brazing process toeliminate voids in the braze joint. In some embodiments, the nichromewire is bent to create spring tension that applies pressure to theassembly during brazing.

A clamping fixture in accordance with an embodiment of the presentinvention applies pressure to an assembly comprising an HOPG portion anda metal portion while the assembly is brazed. One embodiment of theclamping fixture includes a first block positioned on a first side ofthe assembly and a second block positioned on an opposite side of theassembly. The blocks may be made from steel. A tensioning device appliespressure to the two blocks to compress the HOPG and metal assemblybetween them during the brazing process.

In one embodiment, the tensioning device comprises one or more loops ofnichrome wire wrapped around the blocks and the HOPG assembly. In someembodiments, the nichrome wire is bent to create spring tension forincreased pressure during the brazing process. In another embodiment,the tensioning device comprises a cantilevered tensioning spring thatapplies pressure to the HOPG assembly.

In some embodiments of a clamping fixture, at least one sheet of boronnitride felt is applied between one of the blocks and the HOPG assemblyso that any excess braze alloy that squeezes out will not bond to theblocks of the clamping fixture.

The invention is described in detail below with reference to theappended sheets of drawings which are first described briefly below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-1 c depict a copper vane for a microwave amplifier typical ofthe prior art showing signs of cavitation erosion;

FIG. 2 depicts a block of HOPG being machined by EDM techniques inaccordance with an embodiment of the present invention;

FIG. 3 depicts a magnified view of an HOPG magnetron in accordance withan embodiment of the present invention;

FIG. 4 depicts a brazed copper and HOPG magnetron in accordance with anembodiment of the present invention;

FIG. 5 depicts a milling operation performed on HOPG in accordance withan embodiment of the present invention;

FIG. 6 depicts a pyrolytic graphite structure and other materials beingprepared for brazing in accordance with an embodiment of the presentinvention;

FIG. 7 depicts several microwave components in which HOPG structures arebrazed to copper structures in accordance with an embodiment of thepresent invention;

FIGS. 8 a-8 c depict a test structure exhibiting the resistance to brazecycling of a braze joint of metal to HOPG in accordance with anembodiment of the present invention;

FIGS. 9 a-9 e depict examples of brazed HOPG microwave structures inaccordance with an embodiment of the present invention;

FIGS. 10 a-10 b depict a forward wave cross field amplifier vaneincluding an HOPG tip accordance with an embodiment of the presentinvention;

FIG. 11 depicts an alternative embodiment of a forward wave cross fieldamplifier vane including an HOPG tip in accordance with an embodiment ofthe present invention;

FIG. 12 depicts a brazed HOPG test structure in accordance with anembodiment of the present invention;

FIG. 13 depicts an HOPG insert in accordance with an embodiment of thepresent invention;

FIG. 14 depicts a copper CFA vane with an HOPG insert in accordance withan embodiment of the present invention;

FIG. 15 depicts an alternative embodiment of a copper CFA vane with anHOPG insert in accordance with an embodiment of the present invention;

FIG. 16 depicts still another alternative embodiment of a copper CFAvane with an HOPG insert in accordance with an embodiment of the presentinvention; and

FIG. 17 depicts additional views of the copper CFA vanes having HOPGinserts depicted in FIGS. 15 and 16.

FIGS. 18 a and 18 b depict a copper vane milled out to leave a thincopper web to support the pyrolytic graphite insert, in accordance withan embodiment of the present invention.

FIGS. 19 a and 19 b depict an embodiment of a special fixture designedto apply pressure to the vane assembly during brazing, in accordancewith an embodiment of the present invention.

FIG. 20 depicts two additional embodiments of fixtures using bimetallicspring tension and cantilevered springs, respectively, to apply pressureto the assembly during brazing, in accordance with an embodiment of thepresent invention; and

FIGS. 21 a, 21 b, and 21 c depict a sectioned copper vane assembled andbrazed in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates to reliable microwave components withdesirable properties manufactured from HOPG. Graphite generallycomprises multiple layers of carbon atoms arranged in planar hexagonallattices. In its highly oriented pyrolytic form, the hexagonal latticesheets have an angular spread of less than one degree. This structureresults in properties that are highly desirable for use in vacuumelectron devices. For example, HOPG is 100% theoretically dense, justlike diamond, and thus is well suited for use as a vacuum barrier. Whenused to manufacture components that form part of the vacuum seal, HOPGmaintains vacuum integrity. HOPG also possesses the lowest sputteringrate of all materials. Thus, electrodes made from HOPG will emit farfewer contaminating trace elements during operation than will copper ormolybdenum electrodes. Further contributing to this property is thatfact that HOPG has an extremely high melting point. It is refectory andchanges state at a temperature of 3650° C. as compared to copper'smelting point of just 1080° C. HOPG is thus grown on a graphitesubstrate in reactor vessels at temperatures of up to 3000° C., andcontaminates are simply precipitated out, resulting in an extremely purefinished product.

HOPG also exhibits a very low ion erosion rate compared to copper ormolybdenum. For microwave devices that exhibit failure modes due to ionerosion, HOPG dramatically improves operational lifetime. HOPG alsoexhibits a very low vapor pressure, which reduces electron ionization ofthe residual gas in vacuum electron devices. It is likely that gasionization, ionization of sputtered elements, and secondary electronyield are responsible for presenting charge that is out of favorablephase to the output electrodes of vacuum devices, resulting in degradedoperation. In particular, this out-of-favorable-phase charge collectedat the output electrodes manifests as spurious RF output noise.Electrodes made from HOPG will thus result in vacuum devices thatexhibit lower RF noise at comparable operating conditions when comparedwith standard devices employing copper or molybdenum electrodes.

HOPG also possesses a very high thermal conductivity close to that ofdiamond and at least four time greater than that of copper. This enablesHOPG components to dissipate far higher thermal loads before exhibitingthermal damage. This enables vacuum electron devices to be designed forand to operate at much higher power densities.

Because x-ray radiation produced by electron bombardment is a linearfunction of the target's molecular weight, HOPG, which is just carbon,inherently produces less than half of the radiation of a device usingcopper electrodes. It is true that HOPG's lower density will alsoprovide less shielding of the x-rays that are produced. However, formany applications, such as electrodes for klytstrons and thyratrons, theanode line-of-sight emission will be reduced.

HOPG is also extremely geometrically stable with respect to operatingtemperature and exhibits a coefficient of thermal expansion lower thanthat of silica. Vacuum electron devices requiring close mechanicaltolerances as a function of temperature thus benefit enormously fromHOPG components. In addition, the flexural strength of HOPG actuallyincreases with temperature. This, in conjunction with its low mass,enables the design of very strong isolated components with improvedtolerance of shock and vibration.

The spectral thermal emissivity of HOPG approaches 1, i.e., its behaviorapproaches that of an ideal blackbody. Designs that include electrodesmade from HOPG operating at elevated temperature thus benefit from theincreased thermal radiative heat transfer. Furthermore, the volumetricheat capacity of HOPG increases with temperature. Thus, resistance toarcing damage actually increases as the temperature of electrodes, suchas anodes or cathodes made from HOPG, is increased.

Nevertheless, despite these desirable properties, HOPG has not beencontemplated for use in vacuum electron device components because ofconcerns about its structural properties, brittleness, cost, anddifficulties in handling. For example, many vacuum electron devicecomponents require micromachining that is preferably performed usingelectrical discharge machining (EDM). However, due to its resistance toion erosion, HOPG is difficult to process using EDM. FIG. 2 depicts ablock of HOPG 202 that has been subjected to EDM via electrode 204.Extreme erosion of the electrode can be observed as indicated in region206. Nevertheless, the precision micromachined structure 208 visible inthe figure shows that EDM will work in principle for HOPG, despitegeneral skepticism regarding the use of EDM for materials with a highresistance to ion erosion and arcing damage. FIG. 3 is a close up viewof the micromachined structure, suitable for use as a 35 GHz magnetron.FIG. 4 illustrates the finished magnetron device in accordance with anembodiment of the present invention, brazed inside a copper body by thetechniques described in more detail below.

FIG. 5 shows that despite conventional expectations about thebrittleness of graphite, HOPG can be machined using a diamond blade andactually exhibits remarkable resiliency. The HOPG block 502 depicted inFIG. 5 has been milled using a standard diamond blade to cut a channel504 leaving a web 506 just 0.005 inches (5 mils) thick.

HOPG can also be successfully brazed to common materials used in theconstruction of vacuum electron devices, such as Kovar, copper,molybdenum, and copper-clad molybdenum by heating in a vacuum ornitrogen-atmosphere furnace. FIG. 6 shows pre-assembly brazing testsamples in accordance with an embodiment of the present invention,including a pyrolytic graphite structure 602, and sample strips of Kovar604, molybdenum 606, copper 608, and copper-clad molybdenum 610. FIG. 7illustrates post-brazing samples showing HOPG structures 702, 704, 706,708, and 710, brazed to copper vacuum electron device components.

The quality of the braze joint can be seen more clearly in FIGS. 8 a-c,which depict a greatly oversized backward-wave cross field amplifier(CFA) vane structure in which cooling tube 804 is bonded to a vane 802made from HOPG, in accordance with an embodiment of the presentinvention. The structure pictured was braze cycled multiple times andexhibits excellent resistance to delamination, as can be observed. FIGS.9 a-e depict additional examples of components in accordance with anembodiment of the present invention that includes HOPG structures brazedto metal structures made from copper and molybdenum.

FIGS. 10 a-b depict a forward wave cross field amplifier (FWCFA) vane1002 made from copper that includes an HOPG tip 1004 brazed in place.The HOPG tip provides improved thermal conduction, enhancing theperformance of the cooling vane. FIG. 11 similarly shows a copper vane1102 to which a larger HOPG tip 1106 has been affixed by active metalbrazing. Note that this vane also includes a cooling tube 1104 brazedinto a channel cut in the vane 1102.

FIG. 12 shows a small test sample of HOPG 1202 attached to a copper body1204 by active metal brazing techniques using a non-silver bearing alloyof 35% gold and 65% copper. This test sample was then lathe turned andpolished three times and inspected for voids. None were found.

FIGS. 13 and 14 depict a CFA vane constructed in accordance with anembodiment of the present invention. FIG. 13 shows an HOPG wedge insert,tapered and machined to fit inside a copper vane structure. FIG. 14illustrates how the HOPG insert 1402 would slide into the copper vaneframe 1404. Once the insert 1402 is properly located within the frame1404, the entire assembly would be brazed. The high thermal conductivityof the HOPG insert 1402 provides superior thermal performance.

FIGS. 15 and 16 depict alternative embodiments of a CFA vane with anHOPG insert. In this case, the copper vane 1502 includes a milled cavity1506 into which the HOPG insert 1504 is placed and then bonded bybrazing. The embodiment of FIG. 16 is similar with a slightly largercavity milled in the vane 1602 for accepting the HOPG insert 1604.Embodiments such as those depicted in FIGS. 15 and 16 may provide foradditional mechanical strength by allowing a copper web to remain inplace. Additional views of these embodiments are provided in FIGS. 17a-c.

FIGS. 18 a and 18 b depict the same vane design as that shown in FIGS.15 and 16. FIG. 18 a shows a vane 1802 made from copper that has beenmilled out leaving a thin copper web 1804. FIG. 18 b shows a block ofHOPG 1806 inserted into the milled cavity in the copper vane 1802. TheHOPG insert must be brazed to the copper vane, but a difficulty thatarises stems from the fact that HOPG and copper have differentcoefficients of thermal expansion, and even with very precise machiningtolerances, a large average gap of up to 0.006 inches develops when theassembly is brought to brazing temperature. Prior designs relied onadditional braze filler wire on the top surface of the vanes to fill thevoid. While that method can work, it is prone to uneven and incompletegap loading. In addition, excess braze alloy can squeeze out and bond tothe braze fixture. The improved process comprises calculating a precisevolume of additional braze alloy and placing it at the base of the coreand then securing the assembly in a special braze fixture. FIGS. 19 aand 19 b depict an embodiment of a special braze fixture in accordancewith the present invention that produces uniform gap loading free ofvoids and overcomes the problem of braze alloy bonding to the fixture.FIG. 19 a depicts the vane assembly 1902 positioned on top of a steelfixture block 1904. The HOPG block is inside the vane assembly alongwith a precise volume of braze alloy, and a thin copper plate 1906 isplaced on top to close out the assembly. In the event that any brazealloy leaks out during the brazing process, a thin layer of carbon felt1908 is placed between the assembly and the top of the fixture toprevent the alloy from bonding to the fixture.

In order to assure proper flow of the braze alloy through the gap whilethe alloy is in a liquid phase, the fixture depicted in FIG. 19 bapplies pressure to the assembly to force the Pyrolytic Graphite asclose as possible to the base of the cored vane. In the embodiment shownin FIG. 19 b, the fixture comprises a lower steel block 1904 and anupper steel block 1912. They are sandwiched around the vane assembly1914 and secured with nichrome wire 1912 to make a bimetallic fixture.Nichrome wire exhibits a thermal expansion coefficient different enoughfrom copper and steel to ensure that the assembly is maintained underpressure even when the assembly is brought to braze temperature. Thepressure applied by the fixture forces the braze alloy through the gapsbetween the pyrolytic graphite and the copper and effectively eliminatesvoids.

FIG. 20 depicts two additional successful embodiments of the fixturedesign. The fixture shown at 2002 is very similar to that depicted inFIG. 19 b. However, in this case, the nichrome wires are bent to createspring tension to more effectively maintain pressure on the vaneassembly during brazing. The other fixture embodiment includes acantilevered tensioning spring 2006 for applying pressure to theassembly 2008 during the brazing process. The embodiment shown includesa steel base 2004 and one assembly 2008 in place for brazing, but thefixture can accommodate up to six assemblies if desired. The bimetallicfixture and cantilevered spring fixture are preferred over weightedfixtures because weighted fixtures require large size and acorresponding large thermal mass.

Embodiments in accordance with the present invention that supply theexcess alloy from underneath the Pyrolytic Graphite also solve theproblem of having excess alloy in proximity to the fixture. When excessalloy is allowed to contact the fixture, it invariably bonds to thefixture, due to the principles of chemically active brazing. Someembodiments of the present invention may employ boron nitride to preventbonding of the fixturing. And to produce uniform fixturing pressure,carbon felt may be layered within the fixturing in some embodiments, asshown in FIG. 19 a, for example, in accordance with the presentinvention.

FIGS. 21 a-c depict a cross sectioned vane designed and assembledaccording to an embodiment of the present invention. The finishedassembly 2102 is shown in FIG. 21 a. Two cuts were made across thedevice to reveal the inner structure as shown in FIGS. 21 b and 21 c.The HOPG core 2104 is visible inside the assembly 2102. It can be seenthat the HOPG core is uniformly bonded to the copper structure and thatthere are no visible voids.

While numerous specific examples of HOPG components for vacuum electrondevices are discussed above and illustrated in the attached figures,these are provided only as examples and are not meant to limit the scopeof the invention. One skilled in the art will recognize that HOPG may beused for many other microwave components that would benefit from itsdesirable properties as discussed above. These additional applicationswould also fall within the scope and spirit of the present invention.

The invention is further defined by the following claims.

What is claimed is:
 1. A component for use in a vacuum electron devicecomprising: a first portion constructed from highly oriented pyrolyticgraphite (HOPG), the first portion having a high thermal conductivity; asecond portion constructed from a metal; at least one braze jointoperatively joining the first portion to the second portion, wherein:the at least one braze joint provides both mechanical and thermalcoupling of the first HOPG portion and the second metal portion.
 2. Thevacuum electron device component of claim 1, wherein the second portionis constructed from at least one of copper, molybdenum, kovar, and acopper-molybdenum alloy.
 3. The vacuum electron device component ofclaim 1, wherein the braze joint comprises a gold-copper alloy.
 4. Thevacuum electron device component of claim 1, wherein the vacuum electrondevice component exhibits a thermal conductivity that is higher thanthat of a similar component made from metal.
 5. The vacuum electrondevice component of claim 1, wherein the component comprises one of across field amplifier (CFA) vane structure, a magnetron structure, anelectron tube anode structure, and an electron tube cathode structure.6. The vacuum electron device component of claim 1, wherein the secondportion comprises a milled metal structure and the first portioncomprises an HOPG insert sized to fit within the milled metal structure.7. A method of manufacturing a microwave component including at leastone highly oriented pyrolytic graphite portion and at least one metalportion comprising the steps of: machining the at least one metalportion to create a brazing surface; machining the at least one HOPGportion to create a surface closely matched to the brazing surface ofthe metal portion; calculating a volume of brazing alloy that will fitbetween the brazing surface of the metal portion and the closely matchedsurface of the HOPG portion; creating an assembly by applying thecalculated volume of brazing alloy between the brazing surface of themetal portion and the closely matched surface of the HOPG portion;heating the assembly in a brazing furnace to create a mechanical andthermal bond between the at least one HOPG surface and the at least onmetal surface.
 8. The method of claim 7, wherein the step of machiningthe at least one HOPG portion comprises milling the HOPG portion using adiamond blade.
 9. The method of claim 7, wherein the step of machiningthe at least one metal portion to create a brazing surface comprisescreating a cavity in the at least one metal portion wherein the brazingsurface comprises a thin metal web.
 10. The method of claim 7, whereinthe step of creating an assembly further comprises applying a clampingfixture to the at least one metal portion and the at least one HOPGportion such that pressure is applied during the heating step.
 11. Themethod of claim 10, wherein the step of applying a clamping fixturecomprises wrapping the assembly with nichrome wire.
 12. The method ofclaim 11, wherein the step of wrapping the assembly with nichrome wirefurther comprises bending the nichrome wire to create spring tensionthat applies pressure to the assembly.
 13. The method of claim 7,wherein the step of heating the assembly further comprises heating theassembly in a vacuum chamber.
 14. The method of claim 7, wherein thestep of heating the assembly further comprises heating the assembly in anitrogen atmosphere.
 15. The method of claim 7, wherein the HOPG portionis further covered with a metal foil to be bonded when the assembly isheated in the brazing furnace.
 16. The method of claim 15, wherein themetal foil comprises one of copper, copper-molybdenum, molybdenum, andkovar.
 17. A clamping fixture for applying pressure during brazing to anassembly that includes at least one highly oriented pyrolytic graphite(HOPG) portion and at least one metal portion, the clamping fixturecomprising: a first block positioned on a first side of the assembly; asecond block positioned on a second side of the assembly, wherein thesecond side is substantially opposite to the first side; and atensioning device operatively connected to the first block and thesecond block and configured to apply pressure to the first and secondblocks such that the assembly is squeezed between them while theassembly is heated in a brazing furnace.
 18. The clamping fixture ofclaim 17, wherein the tensioning device comprises at least one loop ofnichrome wire.
 19. The clamping fixture of claim 18, wherein the atleast one loop of nichrome wire includes a bent portion that appliesspring compression to the first and second blocks.
 20. The clampingdevice of claim 17, wherein at least one of the first and second blocksis constructed of steel.
 21. The clamping device of claim 17, whereinthe tensioning device comprises a cantilevered tensioning spring. 22.The clamping device of claim 17, further comprising boron nitride feltpositioned between the first block and the assembly to prevent excessbrazing material from bonding to the first block during brazing.